Stereoscopic Measuring and Mapping

Costs for the high-end technologies for stereoviewing briefly introduced above are usually in the four-figure range, and simple 3D viewing and interpreting of relatively oriented stereopairs probably do not merit investing in them. However, they are widely used in digital photogrammetry and 3D visualization of virtual GIS models.

Stereomapping tools work with absolutely oriented stereopairs—the interior and exterior orientations of each image need to be known. They may be determined either by camera calibration and direct measurements during image acquisitions (hardly applicable for SFAP) or as a result of photogrammetric triangulation. The techniques for manual measuring and mapping from stereomodels have already been discussed in Chapter 3.4.2. Using appropriate stereoviewing technology, the operator can collect measurements and features in 3D view, usually storing them in common GIS vector file formats. Stereoscopic mapping as opposed to mapping from 2D imagery has three major advantages.

• Result is distortion-free because the floating-mark positions are automatically converted to object coordinates.

• Terrain height information (spot heights, contour lines)

may be mapped.

• Identifiability and delineability of three-dimensional objects are much improved.

Figure 11-22 is an example for a large-scale topographic map of the gully also shown in Figure 11-1. It was digitized with IMAGINE Stereo Analyst using a photogrammetric block file created with Leica Photogrammetry Suite and prepared for cartographic visualization with ESRI ArcGIS. This map could not have been compiled in the same detail from the already presented 2D imagery because features like the drainage lines, the gully edges, or the heaps of soil material broken off the gully walls are difficult or impossible to identify and delineate precisely without depth information. The distribution of collapsed gully wall material and vegetation cover within the gully provides indications of recent and past gully development, helping to

look back in time from this first image of an ongoing monitoring project (see also Gimenez et al., 2009).

One problem associated with stereoviewing SFAP image blocks is that the variations in scale and orientation are often higher than usual with traditional aerial photography—even kite aerial images intended to be "vertical" may easily be tilted 5-10° from nadir, and differences in flying height of a few meters between consecutive images also are not uncommon. Depending on the stereoviewing software used, this may result in the whole stereomodel appearing tilted and distorted, which is difficult to view. Choosing stereo-pairs with strong tilt or >5% scale difference should therefore be avoided. Quick-check techniques like the one shown with Figure 11-19 are helpful for looking through a large image series.